By vitamins one understands substances necessary for life, that the organism must take up from foods. With a balanced diet, the human daily requirement for vitamins is fully satisfied. In the case of an inadequate or faulty diet, or a resorption disruption, a vitamin deficiency can arise and lead to diseases such as scurvy, beri beri, night blindness and even death. Dietary supplement products such as soya milk for infants are therefore supplemented with vitamins. Foodstuffs are also otherwise supplemented with vitamins. The biological activity of vitamins is structure dependent; it is indicated in weight units or international units (IE or EU). The vitamin content of a sample is, however, difficult and time consuming to determine. This applies in particular for the water soluble vitamins and the trace vitamins B12, folic acid and biotin.
One can determine the vitamin content in a substance mixture (i) by means of chemical-physical methods, e.g., by means of high pressure liquid chromatography (HPLC). These are too insensitive for trace vitamins such as vitamin B12, folic acid and biotin. (ii) With immunology methods. They are not precise enough for many determinations, because of matrix effects, and must be adapted to each sample matrix. Thereby there readily arise interferences with other matrix components, in particular in the case of foodstuffs for infants, feedstuff for cats, serum and blood. (iii) Further, one can determine the vitamin content in animal testing, which is not relevant in practice, and (iv) by means of microbiological methods. Here, there is grown a microorganism, for which the vitamin to be determined is essential, in a specifically deficient nutrient solution, supplemented with a sample or vitamin standard, and the growth thereof or its metabolism are measured at intervals, for example by titration, gravimetrically, by turbidity or nephelometrically. In relation to the background of microbiological determination the following further publications are to be mentioned: GORIN, G. et al. Appl. Microbiol., 1970, 20, 641-642; KELLEHER, B P et al., J. Clin. Pathol., 1991, 44(7), 592-595; BUI, M. H., J. Vitam. Nutr. Res., 1999, 69(5), 362-366; MOLOY, A. M. et al., Methods Enzymol., 1997, 281, 43-53. In microbiological determination one must apply a plurality of dilution series so that at end of the incubation time a growth or metabolic value lies in the measurement range of the parallel standard concentration series. For each testing, a standard curve valid only for this attempt has to be prepared. Further, for reasons of safety and precision, each concentration stage of the standard series and the sample series is to be applied at least three times. The vitamin content of the sample is then determined by means of comparison with the known vitamin content of the parallel standard series. Generally valid precision indications are not possible; the coefficient of variation should, however, lay at around 10 percent or below. The microorganism suspension for the inoculation of the standard and sample series cannot be stored and must be newly grown for each growth testing. There is thus always the uncertainty as to whether the inoculation suspension has been correctly grown or the microorganism has the desired sensitivity and specificity. The microbiological determination of vitamins is very labor and time demanding, and it requires considerable laboratory organization. Only very few laboratories in the field of foodstuffs have specialized in this kind of analysis. Laboratory doctors have ceased to use this method in human diagnosis because of the great outlay, although until today no equally good method for the determination of the biologically active vitamin content is available. Microbiological determination is, however, as before the reference method for all other methods.